This invention relates to a method and apparatus for implementing a trapping operation on a digital image during image processing. More specifically, the invention is directed to implementation of a trapping technique to prevent visual gaps or overlaps between colors on a poorly registered image that is produced by an imaging device, e.g. a digital color printer or copier. The invention is most advantageously used in an imaging device that incorporates therein a split-level buffer.
While the invention is particularly directed to the art of performing trapping operations on a digital image in a system having a split level buffer, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used in any digital image process where trapping is desired.
By way of background, in generating color pictorial images, a large number of colors and moderate spatial resolution levels are generally required to achieve a high-quality image. Because the eye can detect approximately 100 intensity levels, i.e., for three-color separations, seven bits per color separation per pixel, imaging systems should support at least this number of intensity levels. Generally, however, imaging systems support 256 different intensity levels. The 256 intensity levels supported by an imaging system performing three color separations for a full-color image correspond to eight bits per color separation, i.e., twenty-four bits per pixel. Accordingly, for high-quality renditions of real-life scenes, an imaging system supporting at least 100 intensity levels detectable by the eye requires less than 200 pixels per inch to achieve an image having a sufficient level of spatial resolution.
When material such as textual material and synthetic graphic material is being imaged, the accuracy of color is not nearly so important to achieve a high-quality image, particularly since the color used is generally a constant black. High spatial resolution is, however, needed to provide images having crisp, clear edges.
Known devices have been developed to address the issues involved with the provision of crisp, clear edges. For example, a frame buffer for use in a color imaging system includes a plurality of pixels having a first resolution level. A plurality of bits is provided for each pixel so as to enable accurate pictorial imaging. The frame buffer includes pixels having a resolution level that is higher than the first resolution level. Pixels on the edges of objects being imaged are replaced by the higher resolution pixels to provide images wherein object edges have high-resolution while object interiors have moderate resolution. A single, split-level frame buffer is used so that images having more than one level of resolution do not require the performance of separation and merging operations.
More specifically, referring now to the drawings, and particularly to FIG. 1, a frame buffer 10 is illustrated which includes a pixel 12 of moderate resolution which is to be replaced with a block 14 of higher resolution pixels. Frame buffer 10 supports a large color space, e.g., 256 intensity levels or twenty-four bits per pixel for three color separations required to form a full color image.
The replacement of the pixel 12 having a moderate resolution with a block of higher resolution pixels 14 can be effected in a plurality of different manners. For example, FIG. 2A illustrates the division of bits in a four color-separation imaging system, where 31 bits are used to specify a color. As illustrated in FIG. 2B, in this approach, a first bit can provide a flag instructing the system whether tables should be referenced. Accordingly, if the first bit is one, as illustrated in FIGS. 2B-2C, fifteen bits will be used as a table index 30 to a color entry table 32. This color entry table can be used only when two colors are provided in the expanded pixel. The remaining sixteen bits will be used as a reference 36 to a bit map pattern selecting table 34. The bit map indicates which of the two representative colors should be used with each of the high-resolution pixels. This type of approach requires no more memory capability than a frame buffer having only moderate resolution pixels. It is particularly useful for graphical objects and text where only two colors are present (object color and background); however, the approach can also be used when three colors are present, two colors and an intermediate color are present, etc.
These known frame buffers thus enable pictorial material, textual material and synthetic graphic material to be imaged using a single, split-level frame buffer. Object interiors may be imaged at a moderate resolution level while edges that must be crisp and clear in appearance may be imaged at a higher resolution level using only a single framer buffer. High quality images are obtained without requirements of complex configuration and high bandwidth. Different types of material can be processed simultaneously without requiring, for example, pictorial material to be separated from textual and graphic material and combination of the result in a final imaging stage.
Significantly, when an image containing two abutting colors is printed using three or four primary ink separations (CMY or CMYK) on a device that does not have good registration between separations, the result may be gaps or overlaps along the edge between the two colors. A method to reduce this effect is called trapping and involves printing a mixture of the colors along the edge so that some ink will be present even when alignment is poor. It would be desirable to be able to perform trapping operations in the context of image processing systems having split-level image buffers such as those shown in and described in connection with U.S. Pat. Nos. 5,276,532; 5,583,953; 5,644,406; 5,682,249; and 5,684,895xe2x80x94all of which are incorporated herein by reference.
The present invention contemplates a new technique for implementing trapping operations on a digital image that resolves the above-referenced difficulties and others.
A method and apparatus for implementing a trapping operation on a digital image during image processing are provided.
In one aspect of the invention, the method includes determining whether a selected picture element has the edge present therein based on an encoding thereof, determining colors present in the selected picture element based on the encoding, determining whether a trapping operation should be performed based on the colors determined to be present in the selected picture element, determining a trapping color to be used for the trapping operation based on the determined colors, obtaining a bitmap indicating a shape of the edge based on the encoding, and performing the trapping operation based on the trapping color and a one of the obtained bit maps.
In another aspect of the invention, determining whether the selected picture element has the edge present therein comprises checking the status of a flag bit.
In another aspect of the invention, determining the colors present in the picture element comprises accessing a look-up table.
In another aspect of the invention, determining whether a trapping operation should be performed comprises accessing a look-up table.
In another aspect of the invention, determining the trapping color comprises accessing a look-up table.
In another aspect of the invention, obtaining the bitmap comprises generating the bitmap based on the encoding.
In another aspect of the invention, obtaining the bitmap comprises accessing a look-up table based on the encoding.
In another aspect of the invention, the apparatus comprises a frame buffer operative to store a plurality of the picture elementsxe2x80x94the picture elements each being encoded to indicate whether the edge is present in the each picture element, a table having stored therein data on a plurality of colorsxe2x80x94the table being configured such that pairs of the plurality of colors are each associated with a trapping color and the table being selectively addressable based on the encoded picture elements, and a processor operative to determine whether a selected picture element has the edge present therein based on the encoding, determine the colors present in the selected picture element based on the encoding and the pairs of colors stored in the table, determine whether a trapping operation should be performed based on the colors determined to be present in the picture element and the pairs stored in the table, determine which trapping color should be used for the trapping operation based on the pairs of colors stored in the table and the trapping color associated with the each pair of colors stored in the table, and perform the trapping operation based on the trapping color and a one of the generated bit maps.
In another aspect of the invention, a bitmap generator operative to selectively generate bit maps based on the encoded picture elements is provided.
In another aspect of the invention, a second table is provided having stored therein bitmap information, the second table being selectively addressable obtain bit maps based on the encoded picture elements.
In another aspect of the invention, the picture elements are encoded with a flag bit to indicate whether the edge is present.
In another aspect of the invention, the pairs are determined to require trapping based on a comparison of absolute differences to a threshold.
In another aspect of the invention, the trapping color is determined based on an average of the pair associated therewith.
A primary advantage of the present invention is that operations involved with the trapping operation can be carried out efficiently on the tables referenced according to the image buffer encoding, rather than on the image buffer itself.
Further scope of the applicability of the present invention will become apparent from the detailed description provided below. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.